Holographic imaging of a moving object by detecting radiation along a line perpendicular to the object direction of travel

ABSTRACT

An object is passed through a radiation beam along a line and resulting object-modified radiation is detected along a line substantially orthogonal to the object direction of travel. In a preferred embodiment, the radiation is ultrasonic energy and at least one ultrasonic radiation transducer (receiver) is repetitively scanned back and forth along this line in the object-modified radiation as the object moves through the illuminating beam. An electrical output from the transducer is compared with a reference signal that is coherent with the object illuminating beam to generate holographic information of the object in the form of an electrical analog signal. This holographic information electrical signal is utilized to form a hologram from which an optical image of the object may be reconstructed. Any aberrations present in the optical reconstructed image are substantially reduced by illuminating the object with a collimated ultrasonic radiation beam.

O United States Patent [151 3,655,258 Hildebrand [4 Apr. 11, 1972 [54]HOLOGRAPHIC IGING OF A MQVHQG OBJECT BY DETECTING Primary Examiner-DavidSchonberg RADIATION ALONG A LINE Assistant Examiner-Ronald 1. StemAIlOrnEY-WOOdCOCk, Phelan & Washburn DIRECTION on TRAVEL [57] ABSTRACT[72] Inventor: Bernard Hildebrand Seattle wash An object is passedthrough a radiation beam along a line and [73] Assignee; HolotmnCorporation, Wilmington Del resulting object-modified radiation isdetected along a line substantially orthogonal to the ob ect directionof travel. In a [22] Filed: Apr. 20, 1970 preferred embodiment, theradiation is ultrasonic energy and [21] APPL 73 at least one ultrasonicradiation transducer (receiver) is i V V V repetitively scanned back andforth along this line in the object-modified radiation as the objectmoves through the illu- [52] U.S.Cl. ..350/3.5, 340/5 H minating beam Anelectrical Output from the transducer is [5 l] II lt. Cl. 27/00 comparedwith a reference signal that is coherent with the [58] held of Search 6ject illuminating beam to generate holographic information of 1 1/05 lthe object in the form of an electrical analog signal. This holographicinformation electrical signal is utilized to form a holo- [56]References C'ted gram from which an optical image of the object may berecon- UNITED STATES PATENTS structed. Any aberrations present in theoptical reconstructed image are substantially reduced by illuminatingthe object with a collimated ultrasonic radiation beam.

6 Claims, 6 Drawing Figures Patented April 11, 1972 3,655,258

y I24 [2] H6 |I8 MIXER k I23 b R f v ll5 23 sfig sgf 3 4% i? I I N9 '20H9 w; fi J L \F Fig. 1 Fig. 2

MIXER i 1\ I37 I I35 agnar. I

HOLOGRAPI-IIC IMAGING OF A MOVING OBJECT BY DETECTING RADIATION ALONG ALINE PERPENDICULAR TO THE OBJECT DIRECTION OF TRAVEL BACKGROUND OF THEINVENTION This invention relates generally to the art of forming avisible wavefront which is a replica in certain respects of anotherradiation wavefront containing information of an object scene. Morespecifically, this invention is related to holographic radiation imagingof a moving object.

In its broadest sense, the term radiation" is defined as propagatingenergy. The particular form of the energy may be, for example, particleradiation, such as an electron beam, or electromagnetic radiation, suchas light, or acoustic radiation,

such as ultrasonic energy. The invention herein is described primarilyin the environment of ultrasonic imaging since its greatest utilitylies, at the present time, in such an application, but it is to beunderstood that the techniques of the present invention have utilitywhen used with a wide variety of radiation types and wavelengths in theillumination of an object scene. In the broadest sense, the sonicfrequencies utilized in ultrasonic imaging are not limited to anyparticular range but include the entire spectrum of compressional waveenergy. However, in the more practical embodiments of that technique, ithas been found that the higher sonic frequencies (i.e., thoseconsiderably above the audible range) are much more desirable than thelower sonic frequencies. For this reason, instead of utilizing the moregeneral term compressional wave energy in this description, the termultrasonic energy" is utilized.

Visualization of ultrasonic wavefronts or fields passing through objectshas been previously employed to study the properties of such objects andto determine the existence of flaws or defects therein. A continuousultrasonic wavefront is generated by an appropriate source which formsan ultrasonic beam for passing through the object. The object thenmodifies the beam according to its interaction characteristics with theparticular ultrasonic frequency utilized, thereby to form anobject-modified ultrasonic beam which carries information of the object.Visualization of reflective ultrasonic wavefronts or fields has alsobeen employed to detect the presence of objects in a large body of waterwhich cannot be visualized directly. In either case, the ultrasonicenergy beam, after modification by the object, carries informationthereof which is desired to be translated from the ultrasonic to theoptical domain.

The technique of ultrasonic holography provides a more completetranslation of wavefronts than other known techniques. In holography,the phase as well as the intensity distribution across theobject-modified ultrasonic energy is translated into a replica lightwavefront. The phase of the object-modified radiation is compared withreference radiation but is coherent with the ultrasonic beamilluminating the object. This is performed in one manner by interferinga reference ultrasonic beam with the object-modified beam at aliquid-air interface detector to form a standing wave pattern whichdiffracts illuminating light into image carrying diffracted orders toform a three dimensional optical image of the object as viewed byultrasound. The principles of ultrasonic holography are described withrespect to this particular translation technique in copending patentapplication Ser. No. 569,914, filed Aug. 3, 1966. Refinements of thetechniques therein are disclosed in copending patent application Ser.No. 730,260 filed May 20, 1968, issued June 22, 1971 as U.S. Pat. No.3,585,847.

As an alternative to forming an ultrasonic hologram on a liquid-airinterface, the interference pattern generated by the two intersectingbeams may be detected over a two-dimen sional surface in a manner togenerate holographic information in the form of an electronic signalwhich may then be converted into an optical hologram by properlyexposing photographic film or by display upon a cathode ray tubeoscilloscope. One specific technique for accomplishing such detectionutilizes a substantially point transducer that is scanned over atwo-dimensional surface interference pattern. This technique isgenerally disclosed more fully by Preston and Kruezer, in AppliedPhysics Letters, Vol. 10, No. 5, Pages -152 (1967) and is utilized inthe description in U.S. Pat. No. 3,410,363. A technique wherein asubstantially point source is scanned over a two-dimensional surfacearea relative to the object while a substantially point receiver remainsfixed relative to the object is disclosed to obtain the same results asreceiver scanning in copending patent application Ser. No. 662,736,filed Aug. 23, 1967. Simultaneous scanning of the point receiver and thepoint source both over two-dimensional surface areas relative to theobject and a general apparatus for scanning over a surface area isdisclosed in copending patent application Ser. No. 744,732, filed July15, 1968. Non-holographic rectilinear scanning is shown in one form inU.S. Pat. No. 2,989,864.

In copending patent application Ser. No. 782.582, filed Dec. 10, 1968, atechnique is described wherein the object scene from which holographicinformation is to be obtained is scanned over a twodimensional surfacearea relative to both the source and receiver. In order to obtainholographic information from which an unaberrated image of the objectmay be reconstructed, the object is scanned over a spherical surfacearea having a center of curvature at the actual or apparentsubstantially point receiver or point source.

It is a primary object of the present invention to provide a techniquefor obtaining holographic information of an object that is moving inline relative to its surroundings.

SUMMARY OF THE INVENTION Briefly described, the present inventionincludes generating an object illumination beam from a radiation sourcethat is fixed relative to its surroundings and moving the object underinvestigation in a line through the object illumination beam. Objectilluminating radiation transmitted through or reflected from the objectis detected along a line defined to be fixed relative to thesurroundings and to be oriented substantially perpendicular to thedirection of object travel. The radiation along this line is convertedinto electrical impulses by an array of transducers (receivers) or, in apreferred embodiment, by scanning at least one substantially pointtransducer repetitively back and forth along this line. Scanning atleast one transducer provides high resolution more conveniently than avery large number of transducers fixed along the line relative to thesurroundings. The electronic signal so generated is electricallycompared with a reference signal coherent with the object illuminationradiation to produce an electronic signal carrying holographicinformation of the object. A hologram is then formed directly on filmaccording to one embodiment by exposing a photosensitive material to afixed array of point light sources or by scanning thereover at least ona substantially point light source. The light sources are appropriatelymodulated by the holographic electrical signal. Alternatively, ahologram is formed on film by first displaying a hologram on a cathoderay tube driven by said holographic electrical signal and then by takingan ordinary photograph of displayed hologram. A resulting hologramformed on the photosensitive detector is then illuminated by coherentlight in a manner to produce an optical image of the object. A usefulapplication of this technique involves irradiating the object withultrasonic energy to form a visual image for the detection of flaws inthe object, thereby necessitating that the object, and all sources andreceivers be submersed in an appropriate liquid ultrasonic energytransmitting medium, such as water.

An optical image, so reconstructed, requires that appropriate optics beplaced in a light beam diffracted by the optical hologram to correct forastigmatism and unequal magnification of the reconstructed image in itstwo lateral directions. However, it has been found that if the radiationstriking the moving object is collimated, the correcting optics areunnecesary because a good image is reconstructed directly from theoptical hologram.

For a more detailed understanding of the various aspects of the presentinvention, the following detailed description of a few specific formsthereof may be read in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the improved techniquefor holographic imaging of a moving object utilizing receiver scanningalong a line;

FIG. 2 illustrates a modification of the technique illustrated in FIG.1;

FIG. 3 demonstrates area receiver scanning to generate a hologram thatis equivalent of that obtained by the process illustrated in FIG. 1;

FIG. 4 shows a technique for reconstructing an image from a hologramconstructed according to the technique of FIG. 1;

FIG. 5 shows a convenient mechanical implementation of the method shownin FIG. 1; and

FIG. 6 is a modification of FIG. 5 wherein the object includes anendless belt portion that is rotated about a radiation source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The principles of the presentinvention may be understood by reference to FIG. 1. Consider a conveyorbelt 113 moving an object 115 through a holographic object examinationlocation. This object may be a production item desired to be examinedfor flaws by ultrasonic energy. An ultrasonic radiation source 119 fixedrelative to the surroundings illuminates the object on the conveyor beltmoving relative thereto. The source 119 is driven by an electronicoscillator (not shown). A substantially point receiver 121 is scannedback and forth along a line 123 defined relative to the surroundings.The receiver 121 must be small enough relative to the wavelength of theradiation being used to detect phase as well as amplitude information.It has been found that the object motion makes it unnecessary to scanthe point receiver over a twodimensional area. The much simpler linescanning illustrated herein accomplishes the same result.

An electrical output 116 of the receiver 121 is processed by anelectronic balanced mixer 117 wherein a reference electrical signalcoherent with the object illuminating radiation is mixed with the signal116 to produce holographic information in the form of an electricalanalog signal 118. The type of balanced mixer 117 preferred is a phasesensitive detector, and a product detector type has advantages for thisapplication. The function of the mixer 117 is to compare the phase andintensity of ultrasonic radiation detected by the receiver 121 with acoherent reference electrical signal. The reference signal is coherentwith the object illuminating radiation and is generally obtained fromthe electronic oscillator (not shown) which drives the source 119. Theholographic signal modulates a substantially point light source 124which is scanned across a photosensitive film 125 to form a hologramfrom which an optical image may be reconstructed. The light source 124is moved in the y direction across the photosensitive film with avelocity corresponding to that of the receiver 121. The light source 124moves with a velocity in the .x direction which corresponds to thevelocity of the object.

In an alternative form, it has been found that the technique describedwith respect to FIG. 1 may include additional radiation sources, such asan ultrasonic radiation source 120 shown in FIG. 2. An additional sourceor sources may be used to provide additional radiation intensity or toilluminate the moving objects more completely from several directions.In order not to add any distortion to the reconstructed image by the useof more than one source, all sources must be coherent and placed along aline 122 that is substantially parallel to the scanning path line 123.

An analysis of the scanning system illustrated in FIG. 1 may be made byreference to FIG. 3 wherein an equivalent scanning system is shown. Theobject 115 is fixed relative to its surroundings and a point receiver129 is scanned over a two-dimensional hologram aperture 131. Asubstantially point radiation source 133 moves along a line in the xdirection and is at all times in the same relative position along the xaxis as is the radiation receiver 129. To form a hologram, an electricalsignal from the receiver 129 is compared with a reference signal in anappropriate balanced mixer to generate a holographic electrical analogsignal which is applied to a light source 135. The light source 135 isscanned over a photosensitive film 137 following the same pattern of thereceiver 129, or a scaled replica thereof. The developed photosensitivematerial 137 will contain the same holographic information of the objectas will the developed photosensitive film 125. The importantrelationship in source and receiver scanning is between the velocitiesthereof relative to the object, and it may be seen by comparison ofFIGS. 1 and 3 that these velocities may be made the same. For completeequality, the radiation receiver 129 has a velocity component in the ydirection which is equal to the velocity of the radiation receiver 121relative to its surroundings as it moves back and forth along the line123. The velocity component of the radiation receiver 129 in the x"direction, which is also the velocity of the radiation source 133, isequivalent to the speed at which the object 115 travels.

An image reconstructed from a hologram which has been constructedaccording to the techniques illustrated in FIGS. 1 or 3 will bedistorted and have severe aberrations as a result of having unequalvelocity ratios in the x and y directions between the source andreceiver relative to the object. What happens upon reconstruction is,essentially, a reconstruction of two images spatially separated in adiffracted first-order beam, one image representing those lines of theobject parallel to the x direction and the other image representingthose lines of the object in the y direction.

Although such an astigmatic image may be acceptable for certain objectexaminations, it is generally desirable to utilize a corrective opticalsystem during reconstruction of an image. Referring to FIG. 4, ahologram 139 (a developed form of the detector or 137) is illuminatedwith a slightly converging coherent light beam 141 which comes to focusat a point 143. A desired first-order beam 145 diffracted by thehologram 139 brings various points of the image to a focus at separatelocations 147 and 149. One technique to correct for this aberration isto place a cylindrical lens 151 between the two images 147 and 149 sothat the image 147 is transferred into a plane coincident with thatwherein the image 149 lies without affecting the formation of the image149. The viewer then sees an optical image of all object points with aproper relation to one another.

A practical apparatus for carrying out the technique discussed withrespect to FIG. 1 is shown in FIG. 5 wherein rotary scanning isemployed. The specific arrangements disclosed in FIGS. 5 and 6 arespecifically claimed in copending application Ser. No. 820,862, filedMay 1, 1969 by Daniel S. St. John. A conveyor 153 carries a series ofobjects under investigation, such as objects 155 and 157, passed aholographic examination station. A radiation source, 159 which is fixedrelative to its surroundings, directs a beam through the conveyor beltand objects passing therethrough. The object illuminating beam 161 ispreferably collimated, which may be accomplished by the use of anacoustic lens 163. Alternatively, a collimated ultrasonic beam 161 maybe generated by a flat ultrasonic transducer without need for a lens.

A scanning wheel 165 contains a plurality of point radiation receivers,each connected with appropriate information processing electronics(including a phase sensitive detector) to its own point light source ona disk 167. The line scanned by the point receivers is now a curved line169, so it is preferable that the disk 165 be of sufficient diameterthat the length of the line 169 is only a small fraction of thecircumference of the disk, thereby avoiding any severe distortions dueto a curved hologram aperture. To construct a hologram of the objects, astrip film 171 is moved past the rotating point light source carryingdisk 167 at a speed proportional to that at which the conveyor 153 movesrelative to the receiver scanning disk 165. The light sources attachedto the wheel 167 are appropriately focused onto the film 171.

Use of a collimated beam 161 has two distinct advantages over use of adiverging beam. First, the energy of the object illuminating beam isconcentrated and there is no energy density diminution due to adiverging beam prior to reaching the receivers. Secondly, illuminationwith a collimated source has the advantage that a hologram constructedthereby reconstructs directly an image of the object without astigmatismand thereby makes unnecessary correcting optics. A hologram constructedaccording to FIG. 5 with a collimated object illuminating beam 161 isreconstructed according to the technique described with respect to FIG.4 but without need for the correction lens 151.

Certain objects may be more conveniently examined by the techniques ofthe present invention by rotating them about their own axis instead ofmoving them past a holographic examination station by a conveyor-likedrive. Such a modification is illustrated in FIG. 6 wherein thoseelements that are the same as those shown in FIG. 5 are given the samereference numbers. An endless object 162 is rotated about its own axis164. Within the object 162 is a point radiation source 166 which isfixed relative to the surroundings. The object 162 is oriented to rotatein a plane substantially perpendicular to the plane in which thescanning wheel 165 rotates. This technique is especially useful forultrasonic examination of tires which are rotated one at a time as theobject 162.

As will be recognized from the discussion hereinbefore, the holographicinformation recorded on the film 171 will reconstruct an image of theobject 162 that is distorted and astigmatic. This is no problem in mostexamination applications but if it is, the film 171, after development,may be reconstructed as discussed with respect to FIG. 4.

What is claimed is:

1. A method of obtaining holographic information of an object movingrelative to its surroundings, comprising the steps of:

generating a coherent radiation object illuminating beam that isstationary relative to the surroundings, said beam being positioned forthe object to move therethrough thereby to generate object-modifiedradiation,

detecting said object-modified only along a substantially straight linewhich is stationary relative to the surroundings and orientedsubstantially orthogonal to said object travel direction, and

comparing the detected object-modified radiation with the phase andintensity of a reference waveform that is coherent with the objectilluminating radiation, whereby holographic information of the object isobtained.

2. The method according to claim 1 wherein the step of detecting saidobject-modified radiation along a line includes repetitively scanning atleast one substantially point radiation receiver back-and-forth alongsaid line.

3. The method according to claim 1 wherein the step of illuminating saidobject includes placing a plurality of sources along a line that issubstantially parallel to said defined detecting line.

4. A method according to claim 1 wherein the step of generating acoherent object illumination beam includes generating a substantiallycollimated radiation beam.

5. In a method of obtaining holographic information of an object movingrelative to its surroundings through a coherent non-collimatedilluminating beam that is stationary relative to the surroundingsthereby to generate object-modified radiation, detecting theobject-modified radiation only along a substantially straight line whichis stationary relative to the surroundings and orientated substantiallyorthogonal to the direction of movement of the object, comparing thedetected object-modified radiation with the phase and intensity of areference waveform that is coherent with the object illuminating beam toobtain holographic information, and recording the holographicinformation to form a hologram, the improvement of reconstructin anon-astigmatic image of the object comprising the steps 0 illuminatingthe hologram with light to produce at least one astigmatic imagecarrying diffracted wavefront, and positioning optics for correctingastigmatism in the path of said at least one image carrying diffractedwavefront, whereby a non-astigmatic image of the object is formed.

6. A method of obtaining holographic information of a mov ing object,comprising the steps of:

moving an object along a path of travel relative to its surroundings,directing at the path of travel an object illuminating beam of coherentradiation that is stationary relative to the surroundings, said beambeing positioned for the object to move therethrough thereby to generateobject-modified radiation, detecting said object-modified radiation onlyalong a substantially straight line which is stationary relative to thesurroundings and orientated substantially orthogonal to the path oftravel of said object, and comparing the detected object-modifiedradiation with the phase and intensity of a reference waveform that iscoherent with the object illuminating radiation whereby holographicinformation is obtained.

1. A method of obtaining holographic information of an object movingrelative to its surroundings, comprising the steps of: generating acoherent radiation object illuminating beam that is stationary relativeto the surroundings, said beam being positioned for the object to movetherethrough thereby to generate object-modified radiation, detectingsaid object-modified only along a substantially straight line which isstationary relative to the surroundings and oriented substantiallyorthogonal to said object travel direction, and comparing the detectedobject-modified radiation with the phase and intensity of a referencewaveform that is coherent with the object illuminating radiation,whereby holographic information of the object is obtained.
 2. The methodaccording to claim 1 wherein the step of detecting said object-modifiedradiation along a line includes repetitively scanning at least onesubstantially point radiation receiver back-and-forth along said line.3. The method according to claim 1 wherein the step of illuminating saidobject includes placing a plurality of sources along a line that issubstantially parallel to said defined detecting line.
 4. A methodaccording to claim 1 wherein the step of generating a coherent objectillumination beam includes generating a substantially collimatedradiation beam.
 5. In a method of obtaining holographic information ofan object moving relative to its surroundings through a coherentnon-collimated illuminating beam that is stationary relative to thesurroundings thereby to generate object-modified radiation, detectingthe object-modified radiation only along a substantially straight linewhich is stationary relative to the surroundings and orientatedsubstantially orthogonal to the direction of movement of the object,comparing the detected object-modified radiation with the phase andintensity of a reference waveform that is coherent with the objectilluminating beam to obtain holographic information, and recording theholographic information to form a hologram, the improvement ofreconstructing a non-astigmatic image of the object comprising the stepsof: illuminating the hologram with light to produce at least oneastigmatic image carrying diffracted wavefront, and positioning opticsfor correcting astigmatism in the path of said at least one imagecarrying diffracted wavefront, whereby a non-astigmatic image of theobject is formed.
 6. A method of obtaining holographic information of amoving object, comprising the steps of: moving an object along a path oftravel relative to its surroundings, directing at the path of travel anobject illuminating beam of coherent radiation that is stationaryrelative to the surroundings, said beam being positioned for the objectto move therethrough thereby to generate object-modified radiation,detecting said object-modified radiation only along a substantiallystraight line which is stationary relative to the surroundings andorientated substantially orthogonal to the path of travel of saidobject, and comparing the detected object-modified radiation with thephase and intensity of a reference waveform that is coherent with theobject illuminating radiation whereby holographic information isobtained.